1. Field of the Invention
The present invention relates generally to protective structures for vehicular engine parts such as engine exhaust manifolds that generate substantial heat and vibration during engine operation. More specifically, the invention relates to fabrication of a protective heat shield applied to such engine parts, and particularly to a method of fabricating a heat shield having a fold-over crimp at its edge.
2. Description of the Prior Art
The exhaust manifolds of internal combustion engines in today's modern vehicles can reach under-the-hood temperatures in the neighborhood of 1600 degrees Fahrenheit. Such high temperatures create significant risks of damage to electronic components sharing under-the-hood space with the manifolds. Thus, protection has been provided for such components by the use of heat shields designed to at least partially cover up and insulate exhaust manifolds and other heat generating components. In some cases, the heat shields have been effective to reduce measured temperature levels to within a range of 300 degrees Fahrenheit.
One recurrent shortcoming with respect to current heat shield designs, however, has been the inability to reduce or attenuate noise down to satisfactory levels. Generally, the insulation layer is normally the center layer interposed between two metal layers, is relatively thin, and has a relatively high density that makes the insulation layer rather stiff. The insulation layer, while often quite adequate to thwart heat transfer at desired values, has been stubbornly insufficient to dampen noise. Unfortunately, the relatively stiff and thin structures for producing heat shields tend to be prone to producing echoes rather than absorbing vibrations and/or noise.
Another shortcoming of known heat shield designs is that the method for forming the heat shield components often leaves the components vulnerable to cracking problems. Known heat shield designs are formed from superimposed sheet metal layers that are typically joined together in a conventional hemming operation, where the outer periphery of one of the layers is crimped over the outer periphery of the other layer. One known method for performing the crimping operation is crush forming. Referring to FIGS. 1 and 2, in the crush forming process, the edge 12 of a heat shield component 14 has a fold over portion or a hem area 16 where the length L of the fold over is generally constant such that the hem reinforces the strength of the edge of the heat shield evenly. The material being formed into the heat shield component is then crushed into the form of the heat shield.
However, the crush forming process generates uneven elongation in various parts of the edge of the heat shield component being formed. More specifically, the higher the curvature or deeper the drawing area in the part being formed, the higher strain experienced in the part. High strain areas exceed cracking limits and may result in an unsuccessful part. Accordingly there is a need for an improved crush forming process for forming heat shields that alleviates potential cracking problems in the hem area.